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Gradient carbon concentration

The relationship between jump rate and diffusivity in Eq. 8.3 can be obtained by an alternate method that considers the local concentration gradient and the number of site-pairs that can contribute to flux across a crystal plane. A concentration gradient of C along the y-axis in Fig. 8.86 results in a flux of C atoms from three distinguishable types of interstitial sites in the a plane (labeled 1, 2, and 3 in Fig. 8.8). The sites are assumed to be occupied at random with small relative populations of C atoms that can migrate between nearest-neighbor interstitial sites. If d is the number of C atoms in the a plane per unit area, the carbon concentration on each type of site is c /3. Carbon atoms on the types 1 and 3 sites jump from plane a to plane (3 at the rate (c /3)T. The jump rate from type-2 sites in plane a to plane (3 is zero. The contribution to the flux from all three site types is... [Pg.170]

EXAMPLE 9.2. Consider the decarburization of a steel having a carbon content of c0 when it is heated into the austenite (y) region and held in air (Figure 9.13). At this temperature the reaction 2C + O2 -> 2CO effectively reduces the carbon concentration at the surface to zero. A layer of a forms at the surface and into the steel to a depth of x. The concentration profile near the surface is shown in Figure 9.14. The concentration gradient is dr /dr = —cjx, where ca is the carbon content of the a in equilibrium with the y. Fick s first law gives the flux, J = — /)dr /dx = Dcjx. As the interface advances a distance, dx (Figure 9.15), the amount of carbon that is removed in a time interval, At, is approximately (Cy—Ca)dx so the flux is... [Pg.79]

Due to bioproductivity processes, carbon is transported from the surface to deep layers of the ocean, where it is re-mineralized. This process maintains the inorganic carbon concentration gradient and preservation of C02 concentration in the atmosphere at a level which is (100-200) 106 lower than it would be without bioproductivity. [Pg.149]

The effects of bubble size and specific areas of heat exchangers on the transient average carbon concentration and bed temperature are presented in Figure 9. It can be seen that the critical bubble size is about 5 cm, which is much smaller than that for the type A combustor. This is because of the relatively small excess air rate used and the large carbon concentration gradient... [Pg.106]

A non-isothermal dynamic model has been developed for a shallow fulidized bed combustor, which can be used to predict, at least qualitatively, the transient and steady-state characteristics of such systems. Parametric studies have been conducted to examine the effects of excess air flow rate, bubble size and carbon feed rate. It has been shown that an appreciable carbon concentration gradient does exist in the bed. This explains why it is necessary to use multiple feed points in large fluidized bed combustors. A surprising result obtained is that the temperature iii the bed is essentially uniform under all conditions studied even though the carbon concentration is not uniform laterally. [Pg.111]

Thus, given the assumptions above, the surface-deep gradient in inorganic carbon concentration (the strength of the biological pump) is determined by (i) the C/P of the organic matter that is exported from the surface ocean, and (ii) the nutrient concentration ([POd ]) gradient between the surface and deep ocean. [Pg.3344]

If this is an adequate description of the real low-latitude ocean, then the strength of its biological pump (the amplitude of the surface-deep gradient in inorganic carbon concentration) is controlled solely by (i) the carbon/nutrient ratio of the organic matter, and (ii) the nutrient content of the deep ocean (see Equation (5)). If the C/P ratio of export production increased or the [PO4 ] of the deep ocean increased, then biological production at the surface would drive an increase in the downward flux of carbon that is not at first matched by any change in the upward flux of carbon dioxide associated with surface-deep... [Pg.3344]

AES showed that thermal treatment of porous material could result in the change of C/Si ratio as compared with initial stoichiometry. Namely, carbon concentration gradient with the carbon content continuously increasing towards the surface was detected in PSC structures annealed under certain conditions. [Pg.190]

Carbonate is an important composition of inorganic carbon in marine sediments. Up to now, the research into inorganic carbon concentrates mostly on the source, distribution, dissolution, and precipitation of carbonate in sediments. For example, in the western South China Sea, the contents of carbonate in the north and mid-southern areas are high, but low in the middle and southeast areas. The distribution characteristics are controlled by terrigenous material supply and are in close relationship with the extent of the shelf and the gradient of the slope. The contents of carbonate are highest in the area... [Pg.90]

In contrast to the situation in atmospheric FBCs experiments in pressurized test facilities have revealed strong temperature gradients within the fluidized bed depending on the fuel feed system. Therefore a model h been developed to simulate the physical situation inside a PFBC combustor [6]. It is compose of two mass balances for both the carbon concentration Cq an t e oxygen... [Pg.2]

The carbon mass balance (2.1) takes into account the mixing of solids and the combustion reaction and is of reaction diffusion type with Neumann boundary conditions, i,e. the value of the normal derivative dCc/dn of the carbon concentration on the boundary is prescribed. The first two terms in the enthalpy balance (2.3) express the enthalpy flux due to the mixing of solids in the bed, the others the flue gas enthalpy flux, the heat sink due to the heat exchanger tubes and the heat source caused by the combustion. The balance is of convection diffusion type with third type Dirichlet-Neumann boundary conditions, i.e. the temperature values on the boundary depend on the corresponding gradients. Finally, the oxygen balance (2.2) considers the oxygen flux in upward direction and the combustion reaction. This ODE is explicitly solvable in dependence of the carbon concentration Cc and the temperature T ... [Pg.3]

In order now to apply the preceding general considerations to a concrete but still relatively simple case, we shall discuss the Fe-Si-C system. We find here a situation in which a component can diffuse locally against its concentration gradient. This is known as up-hill diffusion Austenitic Fe-Si-C consists of a face-centered-cubic iron lattice with carbon on the interstitial sites. The silicon atoms are substituted on iron sites, and so the mobility of the silicon atoms is orders of magnitude smaller than that of the carbon atoms [22]. In Fig. 7-3 are shown the results of an experiment in which two iron cylinders with about the same carbon contents but with very different silicon contents were welded flush against one another and held for 13 days at 1050 °C. The experimental arrangement as well as the carbon concentration (iVc), the carbon activity (flc)> nd the silicon concentration (iVsi) the end of the experiment are shown. [Pg.117]

Figure 7.6 shows the pH (on the left side) and the temperature (on the right side) for the flow inside the cavity. A vortex appears in the center of the cavity due to variations of temperature and calcite concentration. The fluid moves slowly in a free convection cell. The results for the pH and temperature compare favorably with the ones from Genthon et al. [9]. While the pH (and PCO2) decreases with the depth, the temperature increases from 1 (300 K) to 1.4 (420 k). The deflections of the lines are due to the temperature and concentration gradients. The solubility of calcite is a function of the pH of the fluid, as well as of the calcium, Ca, concentration and ions of carbonate concentration. [Pg.177]


See other pages where Gradient carbon concentration is mentioned: [Pg.161]    [Pg.441]    [Pg.118]    [Pg.39]    [Pg.205]    [Pg.114]    [Pg.228]    [Pg.95]    [Pg.100]    [Pg.106]    [Pg.262]    [Pg.3345]    [Pg.4360]    [Pg.81]    [Pg.186]    [Pg.77]    [Pg.223]    [Pg.509]    [Pg.395]    [Pg.148]    [Pg.283]    [Pg.343]    [Pg.156]    [Pg.163]    [Pg.648]    [Pg.106]    [Pg.113]    [Pg.449]    [Pg.450]    [Pg.191]    [Pg.81]    [Pg.100]    [Pg.99]    [Pg.546]    [Pg.217]    [Pg.293]    [Pg.192]    [Pg.300]   
See also in sourсe #XX -- [ Pg.100 , Pg.106 , Pg.110 ]




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